As is known in the art, single crystal and polycrystal silicon wafers can be produced in accordance with known and customary methods. Thus, single crystal and polycrystal silicon wafers can be manufactured by cutting silicon ingots or bricks. The single crystal ingots are e.g. grown with the Czochralski (CZ) method, by slowly pulling a seed shaft out of molten silicon, which is contained in a fusion furnace. The polycrystalline silicon can be produced by heating silicon pieces in a crucible just above their melting temperature. This lets the silicon pieces grow together forming a massive silicon block. This block is cut into bricks. The ingots or bricks are finally cut into wafers with wire saws. However, a saw damage etch must be carried out after the sawing since the crystal defects having a depth of several pm are centersfor the recombination of electron-hole pairs. Customarily, aqueous acidic etching solutions comprising hydrofluoric acid and nitric acid are used for this purpose.
After the removal of the saw damage, the surface of the silicon wafers is texturized by the aqueous acidic etching solution, preferably, until an etching depth of 4 to 5 μm is reached.
Texturizing consists in creating a certain roughness at the surface of a substrate in order to enable multiple reflection of light incident on its surface, thereby leading to greater absorption of the light inside the substrate, i.e., to an increased light-confining effect. The roughness obtained in this way has two complementary effects: the first effect is to reduce the reflecting power or optical reflectivity of the surface; the second effect is to increase the length of the optical path travelled by the incident light inside the substrate. In a photocell, photovoltaic cell or solar cell, the increased light-confining effect gives rise to an increase in the effectiveness with which light is transformed into electricity.
However, the aqueous acidic etching solutions containing hydrofluoric acid and nitric acid frequently etch surface defects such as grain boundaries faster and deeper than other areas of the wafer surface, which is referred to as grain boundary etching in the art. This way, several micrometer deep grooves appear simultaneously on both sides of the wafers thus reducing their thickness drastically at these positions. This deleterious effect causes a considerable mechanical weakness in the wafers which leads to an increased breakage rate.
These undesirable grooves caused by defect etching appear as clearly visible black lines at the surface of the wafers.
Another problem of the aqueous acidic etching solutions containing hydrofluoric acid and nitric acid is their comparatively short bath lifetime which decreases the throughput of wafers and increases the costs of production. To accommodate this, some or all components of a texturization bath are feed into the bath. This often happens over a feed and bleed mechanism.
Thus, as the active components are replenished in the bath, the amount of silicon dissolved in the bath is increasing over time until a certain level is reached. It is believed that the reduced bath lifetime is caused by accumulation of the silicium in the form of hexafluorosilicic acid during the etching process. Only if this challenge is met a production of several 100,000 or millions of wafers in a single bath is possible.
In order to ameliorate these problems and drawbacks, aqueous acidic etching solutions comprising hydrofluoric acid and nitric acid have been developed, which solutions contain various surface active additives.
Thus, the American patent U.S. Pat. No. 6,340,640 B1 or the German patent application DE 197 46 706 A1 disclose an acidic etching solution containing 12 parts by volume of 50% by weight of hydrofluoric acid, one part by volume of 69% by weight of nitric acid and 12 parts by volume of 85% by weight of phosphoric acid. Instead of the phosphoric acid, a carboxylic acid having a higher molecular weight than acetic acid can be used. Additionally, the acidic etching solution can also contain anionic, cationic, nonionic or amphoteric surfactants, which surfactants are not specified nearer.
The international patent application WO 2009/119995 A2 discloses an aqueous acidic etching solution containing hydrofluoric acid, nitric acid and fluorine containing surfactants. Zonyl™ VSO-100 can be used as the fluorine surfactant. However, this fluorine surfactant is a nonionic surfactant.
The Japanese patent 3916526 B discloses an aqueous acidic etching solution containing hydrofluoric acid, nitric acid and a nonionic surfactant such as a polyoxyalkylene alkyl ether such as polyoxyethylene nonylphenyl ether.
The Korean patent application KR 10-2009-0007127 discloses an aqueous acidic etching solution containing hydrofluoric acid, and nonionic surfactants such as Triton™.
The Chinese patent application CN 1865542 A discloses various aqueous acidic etching solutions containing inter alia and nitric acid and ethanol, ethylene glycol, glycerol and n-butanol as the additive.
However, these prior art aqueous acidic etching solutions are not capable of ameliorating all of the above mentioned problems and drawbacks.